In radar applications it is highly desirable, and often necessary, to distinguish a moving target from the surrounding stationary clutter. The techniques to detect the motion of targets on the ground are known as ground moving target indication (GMTI). One such technique is along-track interferometry (ATI). Simply put, Along Track Interferometry (ATI) uses phase to estimate Doppler (which may be translated to a target's range-rate). When processed properly, stationary clutter will exhibit no Doppler shift, this stationary clutter may be subtracted out, revealing the moving targets as distinct from the surrounding clutter.
More specifically, traditional along-track interferometry (ATI) uses two images of the same area formed from the same spatial aperture but formed at different times. In traditional ATI, these images are then conjugate multiplied on a pixel-to-pixel basis. Between images, pixels having only stationary clutter will have the same phase difference. If the phase of each pixel is normalized by that phase difference (so that clutter pixels have identical phase in both images) then a non-zero phase in any of the pixels will indicate either the presence of a mover, noise, or interference. Pixels dominated by noise will have a random phase; these can be eliminated from further consideration by amplitude threshold detection. Clutter biases the target's ATI phase; fortunately, ATI is synergistic with displaced phase center array (DPCA) processing. The previous approach, shown in FIG. 1, combines the two to achieve both clutter cancellation and moving target detection. Once detected, the ATI phase can be used to estimate range-rate, which in turn can be used to estimate the cross-range offset of the target in the SAR image caused by its range rate.
However, this baseline approach has several shortcomings. Minimum detectable velocity is a prime consideration. The traditional approach has limited aperture to work with for any DPCA pair, subject to the constraint of dividing the array at least into thirds, thus setting limitations on the MDV. In addition, the ATI conjugate multiplication of images is effective but does not provide any integration gain—it is simply a phase detector. Accordingly, a need exists in the art for an ATI system that, among other features, allows use of the full aperture, and provides integration gain across all phase centers.